So if we should be using 7 and 9 for marine why is the KHD supplied with 4 and 7 - surely this also can affect measurements?
The KHD reagent brings the sample ph level down below 7 as a function of the test so you want the probe calibrated to the operating range.
Technically, you could calibrate all the probes with just a low (4) and high (9 or 10), but the wider range would result in a little less precision.
On an aside. I have to calibrate my probes again soon. It has been a few months